tbf it would make a big difference to car-less Edinburgh folk if you could put your bike on a bus to Peebles. Not currently possible afaik, you’d need lothian or whoever to lay on a special bus which had bike space. Seems like a workable idea at first glance.
To the OP – you’ll get a lift to Glentress off someone on here, just post up ahead of time. I’ll probably be there tomorrow for example, people go all the time.
Cycling there would mean a long day for most of us. OK if you’re fit I guess – It’s about 25 miles to Peebles from S Edinburgh along a c_nt of an A road.