When going on a training run, I personally tend to measure the route on a map. I used to try to measure really accurately, but have recently decided a rough estimate to the nearest 0.5km or 200m is usually good enough for most training purposes. If you don't have an OS map and measuring thingy, or even if you do,
Google Pedometer can be quite useful. The disadvantage is that it only has roads, not paths, marked.