Umbros are some of the best shorts, for styling. Considering that many soccer players are reputed to freeball during games, wearing similar-length shorts, not much possibility of "visions", I suspect. At least while you're standing or running. Bending over might be a little different, though, as with stretching.
I realize that the issue of stretching prior to running has seen some differing orientations in more recent years. Used to be that you HAD to stretch before you ran, then some research revealed that it didn't help performance nearly as much as it was thought to. Then the orientation became to not stretch quite as much (or extensively) prior to the run, but be sure to do some afterward. For me, doing some calf raises/hamstring stretches are enough BEFORE the run. Afterward, they don't seem to be that important. But when it DOES become noticed is when I'd so calf raises with weights . . . the next morning, if I didn't stretch afterward, I'd be walking on my tip toes first thing in the morning.
So . . . wear the Umbros you like to wear. Skip the bends, stretches, twists and such that you might be doing before the run . . . other than foot-specific movements. Let your body warming up take care of the rest during the first mile or so--which it will, trust me. Then, when you're finished with the run, put on some sweats to stay warm until you get to your wash-off area. Sitting in a luke warm bath tub, relaxing afterward, you can do your "bend at the waist" stretches and such in a much nicer environment of unhurried luxury and relaxation. Plus some muscle massages as you stretch can be done, too, for good measure!