its not to late ,,,and with proper, care easy 500lbs ,,,,takes about six weeks from seedling to plant pollintaion time ,,good luck,,if it makes you feel any better I just started one for ,,the heck of it ,,to see what it produces ,,,so i`ll keep you posted
I would say go ahead and grow. It will be good experience and you can make a few mistakes on plants that don't count. 100 pounds would be a "giant" pumpkin given this late start. In my climate its 9-10 weeks from planting to pollination.