Final question: are you sure that you have the right gear range on your bike? Are you pedalling uncomfortably fast or are the pedals a pain to budge while climbing that hill, even in your hill-est gear? Cassettes and chainrings can be changed quite easily.

Other than, the interval training is best. Don't over do it, or you'll get muscle soreness and have a terrible time commuting. This applies even more to weight training.