I've lived in both FoCo and BoCo for many years so I can offer a little feedback. Bike infrastructure is good in both cities, much better than larger Colo cities nearby (meaning Denver and its 'burbs). There's a huge bike community in both metro areas (Boulder and Fort Collins).
BUT, I think it would be difficult to go completely car-free as many bike routes are along state or county highways. Winter extreme cold and springtime snow storms can make it difficult to bike during winter months as snowplows push snow and ice into bike lanes, making them impassable or dangerous.