To be honest, I think your trainer is doing you a disservice with that workout. Jumping into jogging those 3 minute intervals isn't easy for someone not accustomed to jogging. I was in decent shape when I did my cardiac rehab and even then it took me a while before I was jogging at 3 minute clips. When I started working jogging into my routine I was walking for 4 minutes and jogging for 1 the first outing. Then it was walking for 3.5 minutes and jogging 1.5 minutes and so on until I was finally jogging decent times and distances without having to take walking breaks. I would also recommend that you extend your warmup and cooldown. My warmup before jogging is 5 minutes. I start with a minute walking at 3.5 mph and add .4 mph every minute after for those 5 minutes. Then I am more ready for working up to the quicker jogging intervals that I'll be doing before ending with a 5 minute cooldown where the first minute is 4.5 mph slow jog and then subtracting .5 mph for each minute after until the 5 minutes are up. Obviously, you wouldn't be working with those speeds right away but the theory is the same.