50 mile loop, you can vary the first part to carrington a lot.
http://ridewithgps.com/routes/2187158
Also going through north middleton at the a7 there are great (steep) roads around there, borthwick and crichton castle and around gorebridge.
or at middleton, keep going over the granites down to innerleithen – good road to come back on too.
up to gifford and up redstone rigg to whitadder reservoir and the lammermuirs is good.
a70 west down the pentlands is ok for a “big” road and a good lead to biggar and the rest, or smaller roads back round the east side of the pentlands
tbh though just going and getting lost in east lothian is probably best – tons of small roads, no killer hills that will sneak up on you unplanned..