It's a good complementary activity but - as the saying goes - the best exercise for running is running. It's reasonable to replace easy runs with spinning but if anything what I tend to do is work quite a bit harder spinning than I would for an easy 45 minute run. My thinking is that you can get a bonus high-intensity cardio work-out without punishing the running muscles.
I'm not in the spinning habit at the moment, having changed gyms recently, but even at the height of my marathon training last year (80-90 mpw) I was doing 2/3 spinning classes each week, and they made a refreshing change from the running.
- Without exception, the highest observed HR I've seen have been from 5k-10k races. - I can regularly sustain an average of over 90% (estimated) max HR during a 10k race. - Interestingly, my average HR stats for 5k & 10k races are very similar. I think this is mainly because, even though 5k is more intense, it takes a longer proportion of the race to get up to a high HR before it plateaus. - Whatever the highest HR you've observed, there's always a chance your actual max HR is a beat or three higher.
So basically I think your HR observations are consistent with what I've experienced. It's not unusual to average over 90% max during a 10k so I don't think it's anything to worry about.