Another easy ride from Belco would be to and from Dickson via the AIS and the path east of GDE that takes you to Dickson.
Or up the bike path from the lake near William Slim (past Mckellar) to Gungahlin, or onto the Barton Hwy verge and out to Gold Creek or Hall.
Or take your bike down to the city/National Library and ride around central and/or east basin. West basin has a few more hills for after you get fitter

I am trying to think of a good place with off road trails that are easy to follow. Most of the loops at Stromlo Forest Park have technical bits, some easier than others...
The other week I rode along a trail at Holt that runs next to the road starting from the bushfire brigade and going out to the top of Stockdill Drive. It would be a couple of km each way. I think you could probably ride on gravel trails all the way from Cook to Holt around the edge of suburbia pretty easily?
I've also been exploring the pipeline trail along the Molonglo on both sides of Coppins Xing, but there is no easy way to get to this unless you drive to the gates on Coppins Xing Rd. And there are gates you have to lift the bike over (lots on the downstream side).
Steph