I don't know anything about your medical situation, but I know all about boring, discouraging training. Its difficult to improve every workout. Its frustrating to feel like you are going no-where.
To solve this problem, give yourself a variety of workouts. If you don't do the same thing every time, its easy to break new records. Everything becomes fun.
These are just suggestions. Its important to do only what you can handle. Talk to your doctor!
Session 1 could be your Intervals. Jog 1 minute, then walk 1.5 minutes. Repeat this process 4 times. See how much total distance you can cover in that 9 minute time period. In the future, try to get just a little bit further in the same amount of time.
Session 2. Walk one mile. Keep track of your time. In the future, try to get a slightly better 1 mile time.
Session 3. Run 50 yards, rest 90 seconds. Repeat 4 times. In the future, cut rest times down to 85 seconds. Then 80 seconds.
Session 4. Half mile. Fastest half mile in any combination of jogging/walking. Next session, get a better time.
Session 5. Walk some distance over a mile, untimed. Next session, push the distance a little further.
Rather than banging your head against the wall, trying to constantly get better at one challenge, give yourself more challenges. Instead of establishing a record in your first workout, then trying to beat that record the very next workout, give yourself more time to get better! Establish 4 or 5 "bench mark" workouts. That's 4 or 5 productive sessions, THEN you start trying to improve. There is less pressure. Its easy to make progress and feel fresh.
Like Chris and ZEB said, you have to start easy and progress slowly, just a little bit at a time. But if you progress slowly at multiple tasks, you get better fast.