Definitely NOT road bike country, although Chatfield State Park, just about 2 miles northeast, has wonderful road biking - trails or on the road, take your choice. You can connect directly ot Confluence Park in Downtown Denver (25 miles from the Marina), from there north to 104th (another 15 miles), or east to Cherry Creek State Park (about 15 miles from Confluence Park - also wonderful bike riding with a nice 10 mile loop) or West to Morrison (15 miles) or west to Golden (20 miles). Or, go east to Parker, where I live - 25 miles or so, then you can go south 15 miles. All by paved bike trail with crossing perhaps 10 streets. The rest is underpasses or overpasses.

It is easy to do a Century.