KR - As MM said, keep a log of all runs, your pace, HR and how you feel. Also information about hills, wind, etc, helps as HR is the effort you are putting in to a run. If you are running into the wind you are putting more effort in to keep the same pace (sorry if that is stating the obvious). On windier days, or days when you are not on top form, your HR can be higher than normal, so making notes helps you remember the sessions. For instance, today my HR is actually lower than the last few times I have done the same run as the wind was against me running downhill and at my back coming back uphill. Made pace look quicker and HR lower 
Like Pete, I rarely do actual sit-ups. I find doing simple weights while balancing on Bosu or stability balls help to strengthen the whole body more than isolating each area.
Edited: 28/12/2011 at 17:13