jump on the first tube on a monday morning.... ride for 5 -6 hrs ending up back at hers, or jump on the tube agian to get home.
You can go to epping, (I ride from Leytonstone / woodford - castle@eight on a sunday morning) or NW to the chilterns, or jump on a train from waterloo to go to surrey hills,
or just ride out to those - 60-90 mins for those on a Sunday morning.
Or get a MTB, jump on the regents canal, head east, then up the lee navigation to epping forrest. I think you can also go west / NW as well from PAddington. Ride 3 hrs out then turn around 
If anyone says ride to richmond park, ignore them.
Hope that helps