I would say listen to "10 Wheels", take it slow and easy with the older bike and maybe adjust it some. I thought 10 miles at first was going to kill me, and now I do 40 without much problem, other than a bad foot problem now that I am trying to buy new bike shoes for.
Good luck and have fun putting on the miles.
__________________
2007 Jamis Ventura Comp
2006 Jamis Explorer 2.0
2000 Specialized Hardrock (bought used)
Swim, Bike, Run and sounds like fun