Hmm, if you’re keen to make the most of your time it’s only 21km of tarmac and what appears to be doubletrack/farm road from Roybridge Station to Luib Chonnal bothy, you could ride that at night for some extra adventure points, although you’d need to be confident in your navigation, bothies can be hard to spot in the dark…
It’s only 60km to Dalwhinnie from there though, so perhaps not really worth splitting into two days, could find accommodation in Dalwhinne then carry on to Aviemore or maybe stop in Newtonmore and head south via the Gaick Pass the next day, catch the train at Blair Atholl…
Anyway, I’ll stop picking holes in my own suggestion for now.