I ran track in college and then later on took up BJJ. I would still hop into the local 5k/10, but noticed if I wanted to shoot for a time in a half or full I needed to cut back on the mat time to focus on running.
It is a good change up from doing the same old running routes and definitely a different type of challenge. Many have said it is a different type of cardio, which is true. However, it can be a nice supplement in terms of anaerobic strength/physical muscular strength.
All in all, I am 43, ran track at an all conference/national qualifier level (participated in the sport for 10 years) and ran pseudo competitively for another 5-6 years post college. I am currently working on attaining a purple belt in BJJ after 8 years of training. I am ok on the mat, but this is the most challenging physical endeavor I have taken on.
If you choose to do it make sure: 1) you find the right gym (some are more recreational or competition based) 2) get good partners that are not out there to "prove themselves" 3) have fun and be willing to be humbled. It can be very emasculating to come across a 130 lb. female that will hand your ass to you.
Hope this will help you make your decision!