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It's a silly question really as I can't imagine I'll get to the junction at ~700m and not want to head up to the summit, if only to maintain the glacial rate at which I'm bagging my final 100 Munros, but will it break me for the remaining 60 or 70km off-road up to Aboyne and back to Edzell on the Fungle Rd/Glen Esk tarmac?
Also am I right in thinking that the descent off the summit, heading north, is big chunky rocky steps?
It's a nice enough day out, but be aware that there are several parts where you really are in the wilderness. Take more food (and tools!) than you think you'll need! There's a lot of pushing as well, which can be hellish if the midges are out. You're right about the descent, but that can be really good fun if you're in the right frame of mind - it's like one of the Mario Kart tracks where you're bouncing from mushroom to mushroom.
Bookmarking for reference, as i have a similar route planned, based on a circuit of Mt Keen and Fungle Rd from Tarfside. And I too am in the final 60 or so Munros
There's a lot of pushing as well, which can be hellish if the midges are out.
Other than the final hike up to the summit?
I was contemplating taking the Mounth road up to Heartbreak Ridge also, what pics I've seen look like potentially rideable singletrack in places... although I think it would be either Mt Keen summit OR Mounth Rd up to Heartbreak Ridge, not both.
What bike will you be riding?
I've only ever hiked Mt Keen (before the path improvements) but it's on the to-do list for the bike at some point. I remember there was a stream which was pretty rapid and required hopping on rocks like a frog to cross. My boss has done it on either his Bird full suss or his Cannondale Lefty hardtail, and a Strava friend has done on his Santa Cruz eBike.
Here's an old thread discussing it.
Quite a few videos on YouTube too.
I think I'd definitely choose HBR over it though (that is as a single thing to do, as opposed to part of a bigger day out).
We did it on mid nineties Kona hardtails so it can’t be that hard.
not much fun either.
We did it on mid nineties Kona hardtails so it can’t be that hard.
Funny, that's why I was considering the gravel bike ("it's just a nineties MTB") but my Superfly with Mezcals is a better choice for any fast or chunky descents or steep rubble climbing.
Thanks for link Citizenlee, consensus seems to be ride the Fungle heading north then Glen Tanar/Mt Keen/Glen Esk coming back. Strange as I thought that was the steeper ascent to the summit and therefore likely to be more hike-a-bike...
I included it on day 1 of a coast to coast bikepacking trip. Pushed most of the way up from the junction and needless to say I pushed a fair bit of the path down til I rejoined the mounth road. If id not been loaded up and a fair few miles in on a hot day I might have managed it. Wouldn't do it again though!
I'd not be doing any of it on a gravel bike! Very loose, chunky, tracks. Last time I did the trail to the summit, it was well maintained but rocky. Rocks were pretty round and smooth, but no way I'd be comfortable descending it on drop bars and gravel tyres. Lots of drainage bars as well. For me, the hardest part of the climb is from the stream crossing after the Queen's well to plateau before the junction. The Zig zags are steep and if you lose traction it is difficult to get going again.
I have only ever came over from the Ballater climb, down into Glen Tanar and that was many years ago, so lots may have changed. No way I'd attempt to go up it. Narrow heather tracks, constantly grabbing pedals and the track across the top was just a series of coloured posts hammered into the ground. I did it after a long dry spell and it was rideable, but can imagine it being a total bogfest any other time. Also consider that even once you are on top of this, there is still a decent amount if climbing left to get to the start of HBR. The exit of HBR takes you out onto the road and it's will add another bit of road riding to get to the Fungle. If you were to attempt this part of the route, take the mtb. HBR would be a waste on a gravel bike.
Other question is why are you starting at Edzell? Would make much more sense to park at Tarfside. If you start from there it is a lot easier to take in the summit as well.
If there is a reason to start at Edzell, there is also no need to ride up Glen Esk tarmac all the way. If you turn up the road at Gannochy, you will come onto estate tracks on the west bank of the river. Follow these up until the bridge just after Dalhastnie, which takes you back onto the glen road for the last section.
Yeah, I think I've already abandoned the HBR idea, just getting carried away! I won't be on the gravel bike, will be on the 29er which seems perfect for these sort of days.
Edzell start was to pick up some extra tracks along the side of the burn, then continue up the west bank of the Esk as you say and finish on the tarmac down the East bank as a nice chilled end to the day. I think I'm now seeing the logic of approaching Mt Keen from the North if it's the climb up from Queen's Well that's the worst bit, I could just reverse and do Fungle first, maybe warm up with the east bank tarmac...
To counter the above is always ride it from. Esk to aboyne and back on the fungle
I cannot fathom any reason why you'd try and ride it the other way.
I think I'm now seeing the logic of approaching Mt Keen from the North if it's the climb up from Queen's Well that's the worst bit, I could just reverse and do Fungle first, maybe warm up with the east bank tarmac...
It’s the worst part of the climb from the south side. Never climbed it from the north, but from the track looking down, I certainly wouldn’t fancy it.
I agree with Terry, do it clockwise. I rather be doing the Fungle later in the day than Mount Keen. It also gives better escape routes that direction if anything goes wrong mid ride. 
Don’t expect the ride down the Glen road to be chilled. On weekends it extremely busy and at any time of the week, the locals treat it as a race track. It’s also a lot more undulating than the map shows. The river tracks are fine as a means to get up the Glen traffic free, but nothing worth going out of your way to ride. They are just grassy Land Rover tracks. 
Start at Tarfside, do the summit out and back and if you want to add some fun, take a detour at Forest of Birse and ride Chutney and Relish before heading back over the Fungle.
I did the clockwise circuit maybe six years ago just as they were rebuilding the path down northwards from the summit. I actually thought the descent was a bit shit and not likely to be improved by the alterations. The Fungle path also rides better S-N as the quality singletrack is then downhill, so I'm in agreement with the OP. The decent S off Mount Keen is mainly doubletrack but would probably be a blast!
We did it on mid nineties Kona hardtails so it can’t be that hard.
I did Mt Keen / Fungle Rd in 2002 on a Dawes hardtail. Next day I phoned the bike shop and ordered the lightest bike they stocked.
We did it in March and some bloke off that-there Singletrack forum turned up at the start to warn us soft-southerners that we'd bitten off more than we could chew. 
Also did Mt Keen in 2007 as part of a coast-to-coast on the light hardtail mentioned above.
PS: Just do it.