There probably is a weak correlation between resting heart rate and running fitness, other things being equal, but there are too many variables affecting heart rate for it to be a good indicator on its own. I started out in running with a good aerobic base and a RHR of around 36. A few years later, RHR is pretty much the same but I'm running 7 minutes quicker for 10k! So my heart's still big and strong but my legs are more running shaped.
Here's my recommendation for a fitness indicator: beats per mile. I use fetcheveryone for my training log and if you input time/distance and average HR it'll work this out for you. Looking back over a year's worth of data it's pretty impressive how well "bpm" correlates to my race performance. When I was returning to running I was struggling to break 1,000 bpm; my record is 904, on an easy run a few days before setting a 5,000m PB. So anything in the low 900s is, for me, pretty good. (Obviously you can't compare across different people because of differences in heart rate ranges and maximums.) Also, compare bpm figures for similar runs; intervals, hill work-outs, etc. will tend to give higher bpm.