Stationary bike vs. on the road

My workouts are similar to LRR. I ususally go between 35-40 minutes with varying resistance. However, it is tough for me to go 10 miles at 18-19 mph. My question is how will this translate outdoors. I realize that hills make a difference, I assume there will be little traffic at races. I was wondering if the coasting effect of the bike outdoors leads to less energy expenditure than the stationary bike.

Melistic do you think you can get a workout on a trainer? I think the bike experts here certainly do. I agree that many people are just spinning at low resistance but if you crank the resistance up the muscle work should simulate riding outdoors. Similarly running on a treadmill works as a substitute for outdoor running. It does not take into account weather, hills, asphalt but I am sure I can get a good worout on the treadmill. I do not necessarily equate treadmill workouts with outdoor running. I think a 7:30 mile on the treadmill is probably 7:40-5 on the road. Hence the question Is there any way I can know my fitness level using the stationary bike due to my inexperience on the road. I want to have clue as what to expect in April when the weather is warm enough to get outside.

Thanks for all the answers. I feel more comfortable in believing that if I push hard on the stationary I will see results on the road. Having said that I think I will also get a trainer. Not that I am planning on kicking ass all over the country. If I could kick any ass here in MA that would suffice. What I really will use it for is getting used to my bike and the convenience of working out at home.