Light/easy? Morning definitely since there's less traffic. Could go through midday, like 1030/11 to 130/2 or so. Start out nice and easy, finish up fast and cardodging. Get the blood going and hit the pub. Sounds like a perfect start to a saturday to me!
I can easily come up with a route to most of the attractions. Touristy or otherwise... 3-4 hrs would definitely get outside of central so maybe Momentum could help out with some Brixton/south of the river action, I could work something out around Angel, Camden, Marylebone/Edgeware maybe even Hackney if ya dare. Care to hit the Limehouse Link tunnel (FAST!) and go out to Canary Wharf? It's the most American-like part of London...

BTW, if any of you guys can get some Cinelli 67-39 bars I will gladly repay you and buy you a few pints.