i managed to run 4 miles today on a treadmill. It took me 63 minutes which is close to what some people can walk, but I was running/jogging.
Same approach. Everytime my pulse got to 150 or 151 I dropped my speed by 0.1 or 0.2 mph. Started at 4.8 then to 4.6 then dropped it to 4.0 where I stayed until 2+ miles then I dropped it to sub 4.0 mph for the rest of the time.
While it was nice to know that I could go 4 miles which I've never done before, I think it might be a better use of my time to work on extending the amount of time that I can go at above 4.0 mph while keeping my pulse under 150 bpm. I can do this in 30 minutes instead of an hour (an hour on a treadmill is a long time even with music and watching cute kids learn soccer) unless I suddenly get a whole lot more fit and it takes longer for my pulse to go over 150. It's problems like these you wish for. The other cool thing was knowing that if I wanted to I could have gone longer.
I do need to replace my shoes as even though they've only been used on a treadmill they are less springy than they used to be.