In reply to pepperpot:
Since I've got a bit older I've started to get a bit more patient, and had wondered about increasing it by a smaller amount each week. There's no rush I suppose and as I'm enjoying it so much the last thing I want is to get an injury.
In terms of intensity I have three usual runs;
1) a loop one which is largely flat with a long section of to canal tow path
2) a linear one that is generally uphill to the turnaround point
3) a random run round a local woods which I make as hilly and as muddy as I feel like - I tend to be less rigorous with heart rate on this one and push myself when it feels good.