Looking for something new to do on the 'mill, I came up wit this...and enjoyed it.
Start at a comfy pace. Every 1/2 mile (or .50 on your 'mill) pick it up .5 mph. So, if you start at 4mph, you'll run that pace for 1/2 mile and then take it to a 4.5 for the next 1/2 mile. Continue adding .5mph every 1/2 mile. I ran 5 miles this way. It's a great way to get a nice warm up jog in and then push it a bit at the end. This is also helpful to practice negative splits...which is a fancy way of saying you get faster as you go.
Oh, if taking it up .5mph at a time is a bit stressful, try just taking it up .2 or .3 mph at a time.
Try it out.