I’ve done a few different ones similar to druidh’s suggestions.
One other option that I’ve used is to come out of the Pentlands around Carlops or Nine Mile Burn and then using farm tracks and minor roads to get to Gladhouse Reservoir area. From there I never found a great direct route south through the windfarm – but it should be doable (?). Less direct you can follow tracks east to the back road above Heriot and Innerleithen, follow the Leithen water down, turn in (west) on the big forest/estate track past the big house and up the obvious climb to pick up the black route near the top.
I’ve extended something similar to this a couple of times to make a route from Edinburgh to Gala vising Glentress and Innerleithen and then back up onto the SUW. It’s a nice day out but again, there’s not much singletrack.