It's not a smooth climb from St Mary's Loch but not too bad. The steepest bit as I remember is by the dam for the upper reservoir. The descent to Talla reservoir is a wee bit scary and all that braking strains the wrists. After that it is a very nice descent along the Tweed until the Meldons.
Potential cafe stops are:
Innerleithen (some good'uns here but best to take a lock)
Loch of Lowes Cafe (requires a diversion of 2 or 3 miles)
Dawyck Gardens (I think we can get in without paying entry)
Peebles (requires a diversion)
Eddleston cafe or pub (the former has awful coffee!)