Maybe I can help. I've got an old, rigid mountain bike set up for road use, a homemade-hybrid, if you will. For reference, I'm 45 years old, I'm 6'2" and weigh 220, and I ride about 75 miles a week.

My bike is very comfortable. Here's what I've found:

1) Get a road seat. Forget wide, spring-loaded, and gel-covered. Those seats are just too wide for "real" riding. Your legs have to go round-and-round, so anything wider than, well, a road seat, will lead to pain and chafing and bad stuff. Look at ANYbody on ANY kind of bike that does long rides and you'll see a small saddle.

2) A small saddle hurts your tail for the first week or so. Then something magic happens inside your body and it doesn't hurt anymore. Really. You just have to commit.

3) A low saddle and a high bar puts all your weight on your tail. A low bar and a proper saddle height spreads some weight to your hands. This is a good thing. The bike handles better, and so does your tail.

So you're doing the right thing following the traditional rules of set up. I say commit to it for two weeks and then get back to us. Expect it to feel way different. Give it a long try.

P.S. - Nashbar sells 26" x 1.25" slicks that will fit your MTB rims and will accept up to 100 PSI. They cost about $12 each. They're sticky and way faster than what you have and are the biggest, single factor in making your old MTB feel like a road bike. I'm on my third set in several years.