Definetly only one way to build up your fitness for cycling and that's to cycle your bike!cycling makes you good at cycling,sounds obvious but so many people try other things treadmills,rowing machines,stationary bike,but these are just side shows to the main event,also riding a bike well is a skill set not just a fitness set.riding will also naturally develop your road craft,pretty important as a non driver,and the london commute involved,I think you'll be amazed how quickly your base level of fitness rises,and your confidence on the commute will just arrive by itself.