I'm 6'3" and have 175 cranks. My average, year to date, is around 79 or so, but that includes some serious climbing when the cadence dropped. I find that riding by myself, I usually hit the low 80's, but get up into the 90's when I'm pushing to keep up with the younger guys in the 20+ mph range. It takes some time and some work to get the cadence up there, but it is worth it. It feels pretty effortless to be riding with a mid 80 cadence, hard in the low 70's and ridiculous when I get over 100.

One thing that you could try to get the cadence up is to spin on any downhill stretches you have. If you can spin at 110 to 120 going downhill, your cadence will improve.

On a side note, I live in South Florida, but was riding in the North Carolina mountains 2 weeks ago. A local hot shot told me he has hit 60+ mph going down mountains and that his downhill speed is limited only by his cadence. He told me he has hit 200 rpm on his trainer and is trying to get that high on the downhills. I find that hard to believe, but what do I, a mere flatlander, know? If I was any gear on flat land and got close to 200, I think I would be hitting 40 to 59 mph. It sounds impossible to me.