Nice speeds @unigoof, particularly for 125mm on a 24! I could not do that and since you can I think you should go shorter for such a small uni.
I looked back at some of my rides on a 26" with 89mm cranks last year. I see one in the lower end of the range of distances you mentioned, where I cycled 16.82km (10.45 miles) with 347m (1138 feet) of elevation and multiple road crossings. I did it in just under an hour, meaning I averaged 17.1km/h (10.6mph), despite all the hills and for 20 minutes I even managed to average 19km/h (11.8mph). If I can do this, I have a feeling you could do better, with the right (shorter) setup.
P.S. In terms of strength and stamina I feel I could probably go faster but the real problem is applying it on a small wheel with short cranks, while remaining stable. So this is probably about as fast as I can realistically go over a course like that, with such a setup.
For those on Strava who are curious: https://www.strava.com/activities/4904310722
My uni setup at the time