In that case, why not enter a 5K race somewhere local to you, that way you would be able to follow a training schedule again with the 'reward' of a race for your efforts. you could then increase the mileage of each race you enter as you continue to improve your distances.
As for last night, don't worry .............. we all get them, I tend to find that I ocassionally have a shocker after a bad / busy day at work and my preparation has not been what it should be (either that or a slight 'mars bar' lapse to the diet

. Are you sure you are not over doing it as well, rest days are equally important as part of a training schedule.
As for improving general cardiovascular fitness, swimming, rowing and cycling are good and also give the legs a break from pounding the streets.