Nice looking route, I struggle to get out on the road bike when I’m in the city as I just can’t be hooped with the roundabouts, traffic and lights around the outskirts.
That said, you could add maybe 5 miles and some great wee hills if you added the Gilston road (a bit ‘farmy’ but good climbs and descents) and then crossed the main road to join the road from Heriot and climb up to the minor road from Inners, turn right and descend to Middleton to rejoin your route.
Might need to try this soon, can’t just keep getting the train to Dunblane every time I want to ride the road bike!