Following a proper warm up, to get maximum value from your treadmill sessions with limited time you should focus on 5k as a distance, varying pace according to the session, according to running coach (and legend) Bruce Tulloh. One of the main benefits of training indoors at pace is the lack of air resistance, meaning you can run faster on a treadmill than on the road, once you get used to it.
The ideal uses of time would be to switch between two sessions; one of your sessions is at threshold pace and the other one is done as an interval session.
For the threshold session -
Do one kilometre at an easy pace, then go straight up to your 10-mile to half-marathon race pace for three kilometres, with a one-kilometre cool-down.
For the interval session -
Start with a one kilometre warm up, then run 3 x 1K at your 10K race pace, with two or three minutes of jogging after each. Finish the session with a one kilometre warm-down.
As you get better, try to build up to 6 x 1K, then increase the pace.