Hi Dan,
First off, remember "Rome wasn't built in a day." So, be kind and patient with yourself on your sojourn back into the ring at the weight you desire. Anyway, here are some thoughts...

You said you're 31 and have been out of the ring (and I assume, out of fighting shape) for four years. Well, that's a long time in both cases. So, it will take some time to get back into fighting trim, which may not be at the weight division you desire to compete in. Even if you do have the desire, dedication, and drive to work your way back to excellent condition, you may not be able to realistically go back down to the middleweight category. Depending on your genetics/metabolism, you may have to settle in at super middleweight or light heavyweight in your comeback. If this is the case, don't beat yourself up over it. We all change as we get older...

You mentioned in your post about going back to your "old training routine." Well, this is something you're probably going to have to revise if you want to maximize the potential you currently have.

Since I don't know what your "old routine" was like, I'll assume it included plenty of long distance runs. If so, ditch them and emphasize more HIIT (i.e. High Intensity Interval Training) regimens using such proven methods like the Tabata Protocol, the Little Method, and Turbulance Training (look them up via Google for more info) instead.

In terms of diet, you probably already know what to avoid, but just as a quick review, simply avoid processed foods of any kind as they contain high levels of sugar and/or salt and/or artificial ingredients that are just not good for you in terms of losing weight and maintaining your health in general. Now, this is easier said than done, so you have your work cut out for you.

Another recommendation I have is to use the diet tracker/calorie counter site "myfitnesspal.com" It's an excellent site that can calculate (based on the info you provide it) the number of calories you can consume per day in order to lose the designated amount of weight you want to shed and do so in a safe, realistic manner. Moreover, it significantly helps you do this by providing constant feedback on what you consume. So, give this a shot.

Another suggestion I have based on what you wrote in your post, is to work with a good trainer, a really good reputable trainer. Nothing beats that on your road back to the ring. And heck, if you can afford it, hire a dietician as well. Again, I don't know you or how disciplined you are/can be, but if you don't have it in you (which may be the case since you let yourself really go over these last four years), it'd be in your best interest to solicit expert help from others who know their stuff. And with your stated desire to get back into the ring for another go-around, I strongly suggest you go this route.

If you're going to do something, do it right and to the best of your abilities.

Anyway, good luck ("luck" meaning when preparation meets opportunity). Keep us posted on your journey back.

Take Care,
Lito