Posted: 03/12/2011 at 21:06
I haven't run a marathon... but I would say you obviously have a very good base of fitness if you are regularly running for 60-120 minutes. I rarely run for as long as 120 and have been running for a few years now.
I would say you would get more benefit from just doing as many miles as possible and maybe looking to extend your long run to 150 minutes and maybe eventually 180 minutes.
I don't think, given your current level of training, that 30 minute jogs would be of any particular benefit...