I found a solution to my little exercise problem; start the workout by following a YouTube aerobics video to get my heart rate up, and then switch over to the treadmill to keep it there for the full ninety minutes if possible, or whenever my legs start to go numb, whichever comes first. If the latter, then I go for another walk later, outside. Much respect to the women doing these aerobic videos for keeping their arms and legs coordinated... to me it's like patting your head and rubbing your tummy at the same time 
... so I'm focusing mainly on the upper body for the ten or twenty minutes I'm doing that, because my legs will get plenty of workout on the treadmill for the rest of the time. It's a pretty crappy, manual treadmill that I have... not really suited to a natural experience of walking, or running for that matter, so I think that's why I can't get my heart up easily on it - I think it would probably be a lot easier on a motorised treadmill - but it's fine for maintaining a heart rate, so I'm happy with this hybrid approach three times a week.
... so I'm focusing mainly on the upper body for the ten or twenty minutes I'm doing that, because my legs will get plenty of workout on the treadmill for the rest of the time. It's a pretty crappy, manual treadmill that I have... not really suited to a natural experience of walking, or running for that matter, so I think that's why I can't get my heart up easily on it - I think it would probably be a lot easier on a motorised treadmill - but it's fine for maintaining a heart rate, so I'm happy with this hybrid approach three times a week.
