The first thing to note about exercise is that everybody has a different opinion and it is more important what works or doesn't work for you in terms of keeping you motivated.
I do a lot of long distance running, and I find the most important thing to keep motivated is to have specific goals like it seems you do (e.g. run a marathon, run a sub 1:30 half marathon). I also like plotting the runs I have done and keeping a record of my times over courses which I tend to repeat, it keeps it interesting to see improvement over a training period leading up to a race. www.mapmyrun.com is good for plotting and logging runs.
Doing a lot of running I also seem to pick up quite a lot of injuries, make sure you have well fitted trainers from a specialist running shop. Also make sure to cross train (also like it sounds like you do) by cycling and swimming to avoid building up large muscle imbalances and more importantly to take some strain off your legs and joints caused by repetitive impact, also run on grass or soft ground as opposed to tarmac or hard paving where possible. Stretch off properly after exercise and make sure to factor rest into your training schedule - Just recently I had to take 6 weeks off and missed out on running my first marathon due to a stress fracture in my foot caused by overtraining, it's incredibly frustrating.
Good luck!
tweehonderd graden, dat is waarom ze me mr. fahrenheit noemen, ik reis aan de snelheid van het licht, ik ga een supersonische man van u maken