Training plan for weightlifter

I have been lifting weights for several years - not to become a bodybuilder - but to keep muscles defined.  When I take some time off from lifting - I lose that definition.  Right now - I lift about 45 minutes and do about 30 minutes of cardio 5x/week.

I am 48 and I'm thinking about getting back into sprint triathlons.  I have done a couple of them about 10 years ago.  I basically have a swimmer build - with a little bit of a gut.  And I hope that adding more cardio work will trim off the fat.  

Which training plan do you recommend if I want to keep the muscle - but get in shape to do the triathlon?

Thanks!