Getting back to the best

Hi there, I gave up competitive swimming age 15 and continued fitness for another 2 years, however in that time i definitely felt that i lost my "speed", is that just because of lack of training or is it age? Im 19 now and figured it would be better to ask those coaches and more experienced swimmers on the route back. Im probably (85kg or 14.5 stone and 6 foot 1) 2-3 stone over weight and haven't even been in a pool in a year. How would any of you recommend getting fit enough to train? Cardio at the gym? Swimming? Ps, I'm willing to give absolutely everything into this, i really dont care if it takes 5-6 hours out of my day 7 days a week, or if i have to wake up at 4 in the morning. I just want to be back competing at the highest level possible. Thanks in advance.