The Antonine wall has just been made a World Heritage site, this is long over due, it is an absolute jewel in places. I don't think that you can walk the entire wall, a motorway to Edinburgh crosses it near Falkirk, but you can walk much of it in the Falkirk area, Rough castle is well worth a visit, and I've been to the part at Kinneil house at Bo'ness also and that was good. Mr Coot has visited the part at Bonnybridge and said that was good also. You might be best combining the Forth and Clyde and Union Canal walk with the Antonine wall, as they run together for part of the way.
Also interesting, if you care to go further north, is the Roman Fort Ardoch at Braco, it is amazing and so many people drive past it on their way to the highlands and it really is worth a visit (and there is a good pub in the village). There is a whole line of Roman fortlets north of Braco in Perthshire, the Gask ridge, many of them are getting bad rabbit damage, this is the northern boundary of the Romans in Scotland and predates Hadrian's wall.