Post c25k/8k/10k - New Goal Needed

Hi All,

I have been an avid cyclist for a number of years (completed my first ms150 this year) and have moved to running in order to loose a bit more weight. I started off first with the c25k and then progressed to 8k and then 10k.. Well now I am 20lbs lighter and went from xl shirts to mediums.. None of my pants fit anymore as I went from a 36-38 to a 32-34.. Running and calorie counting on this site have helped tremendously. Well now I need a new fitness goal. Should I go for the distance half-marathon, try to increasing my speed, or change it up over the winter by starting a swimming program? I feel that my core and legs are in pretty good shape but my arms and chest are seriously lacking since I have never done any sort of weight training.