Easiest way as others have said is Thriepmuir – Green Cleugh and then down the tarmac road past the 2 reservoirs to Flotterstone. Easy on a gravel bike. Green Cleugh / The Howe is a bit rough in places but the rest you could do on a road bike.
The road climb up to the start of Green Cleugh from the Thriepmuir carpark (called Exponential on Strava) is short but quite painful!
There is a river crossing at the bottom of Green Cleugh that used to be shallow and really fun to smash through on a gravel bike, but after the last few years storms it has deepened quite a bit. Would get pretty wet if a rock stopped you and you had to put feet down. I usually get off and hop across the stepping stones these days just so my feet don’t get cold anyway.
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If you don’t mind, my favourite way to get between those two points is the following (need an MTB, and only enjoyable / responsible in the dry really):
Thriepmuir to Harlaw along the track next to the water
>From Harlaw, climb up the “Drainage ditches climb” towards the top of Maiden’s Cleugh (really nice climb, but only if it’s dry)
>From there, traverse left over the northern flank of Harbour hill.
> Follow the track to go up and over Capelaw, down to the big gravel road.
>Follow the road to Fala Knowe, and then drop down a faint path to the right towards the reservoirs. This is awesome fun in the dry.
This kicks you out on the road near Castlelaw Hill fort, and you’re basically at Flotterstone then.