Running when you are me

So because I have reduced cardiac capacity it is really difficult for me to run for a long time. I can't push myself too hard because I get faint and start feeling ill. A few years back I bought myself a Polar heart rate monitor. I use it not only to keep track of my heart rate, but also to help me interval train. At the moment, I can comfortably run for 5km if my intervals are set to 40 seconds running 50 seconds walking. Every week I increase this running time depending on how I am feeling. The confusion comes in about when to increase the distance I am running? Is it better to work on distance, or work on a pace goal first and once I have achieved this then increase the distance? I still have to work out a strategy about how to do that...I've read "regular" beginners training guides and they want you to run for 2min and then walk for 2min... I can't do that, everything has to change. Heartrate related training programs also don't work because my heartrate doesn't vary much, because of my medication. When I'm exercising it usually varies between 130 and 140, when it does increase above that it usually jumps to about 240 which I believe is my monitor picking up atrial beats... and usually means I'm pushing way too hard

But I think the interval training works well and at the moment I am running/walking 5km in just under 40mins, which I don't think is too shabby. I think I will stick to 5km for some time as I feel a 35-40min exercise session is good for me at the moment, any less and I feel a bit like I didn't do enough, any more and I'm exhausted. I was running a bit faster when I was in Bergvliet but I'm sure I can get there again.