Shouldn't be a problem. Mileage will be in the region of 80 miles for the day, with a few hills to tackle (rolling hills rather than major climbs). Just make sure you take the B7016, or the minor roads south of it, from Biggar to Broughton; and not the bridleway that I ended up on!
If you have the time, I would opt for the camping in Melrose option though. This route is quite scenic and passes through some nice towns - Peebles, Innerleithen, Galashiels, and Kelso are all worth spending some time in.