If taking the bike in that direction I always take it as far as the Luibeg burn. There is a boggy section just after Derry Lodge but it is short lived, after that the track is a bit rougher with some drainage ditches but it is fairly straightforward cycling (I'm no cycling expert)
I do agree the camping options are far better at Derry Lodge, or have you thought about camping or staying at the bothy at Corrour making the next day even shorter.
If walking it only takes me about 2 hours to walk from Linn of Dee to the Luibeg burn but it's the way back where the bike is the big advantage with the cycle out from Luibeg to Linn of Dee a breeze compared to the long walk.
All the best.