Running

Strength training. Weights. That will give you the best results since your page says you want to basically stay the same weight.

Get your body fat % measured too. and then work to get that lower if you have to. If you want your abs to show then you have to lose the fat covering them. Cardio will help you burn calories to create the deficit but its not going to build/create the muscle you desire. At least not effectively.

I'm definitely not an expert but this is the advice I've been given and its working for me.

You could go as high as 25/25 for your 50 min workout if you really wanted but more weights will be more effective. Cardio is good for heart and lung health so you don't really want to quit it altogether but definitely focus on strength. If you have the finances, I would recommend a trainer to help you set a routine and get started. Make sure your form is proper so you dont get injured. Exercise should be fun and enjoyable- there is a workout that works for you out there somewhere. Play with it until you find it. You don't need a ton of equipment, just start lifting heavier things.

You will also want to watch your diet too. Most people recommend eating 1g of protein for each lb of lean body mass. Its going to help protect/repair/build muscle. Eating maintenance calories and doing lifting/weights should change your body composition so that you are more muscle and less fat. You have to eat a surplus to gain muscle. Eating too little is not going to help you at this stage so make sure its enough fuel. iifym.com has some good calculators for figuring your TDEE, as does Scooysworkshop.com

Good luck!

if you're new then you shouldnt be doing HIIT. you need to first build up to a base level of fitness FIRST, then start working in intervals. once you have a a pretty good conditioning then add HIIT.

if you're new to running, the same advice applies. first get to a point where you can keep a uniform comfortable pace for a certain amount o time (ie 30 minutes) then start building on your time and your speed by introducing things like speed and hill intervals

Eat at a slight deficit, strength train, and do cardio once or twice a week. Give it time. They will come. Simple and hard as that.