Yesterday I rode my Trek 7.1FX (700x35 tires) 144.3 miles through hills with 80-90F temperatures and about 30 miles of which were in direct head-wind. I hauled over 20lbs of gear with me, too.
The ride was very tiring considering the very few hand positions (flat handlebars) and only having platform pedals. I rode from Springdale Arkansas to visit family in Cookson Oklahoma and back again the same day. It did take me about 10-11 miles of actual riding time. But I guess that's not too bad considering the conditions. And luckily, I had no mechanical issues at all so the bike rode very smoothly.
I definitely realized how badly I want to convert to clipless pedals after a ride like that. And getting some drop-bars might be pretty nice.