That's what group classes are for - to be so busy trying to do whatever the instructor is doing while thinking of how ridiculous you must look in front of everyone else that you can't even think of how much time you're 'wasting'.
Exercise is sometimes boring and painful. You just need to deal with it. Training to music helps and has some science to back it up. But whatever you do it is always going to be boring and painful at times.
I've heard it said but I've got to say I'm struggling with this, I think about what I'm missing out on when I'm exercising, even when I'm weight lifting or using resistance training which are among my favourite sorts of exercise.
So how do you do it? Keep training or fitness interesting and enjoyable? Is it just a second nature thing or innate or is it something you've have to consciously develop?
This is my biggest problem. It's not that I don't understand the benefits of exercise, it's that it kind of bores me and takes time away from things I'd rather be doing. I need a way for exercise to be fun that only requires 1, or perhaps 2 people. I don't enjoy the competitive aspect of sports that much.. it's just not for me.
I've never been able to stick to an exercise regime before. I've been doing yoga for about a year now (by far the longest I've stuck with any kind of exercise routine), and for me the non-competitive nature and focus on looking inward (something my weak Si is definitely in need of) has helped me enjoy it and feel good about myself while doing it without feeling downtrodden by it.
For me anything creative helps, and being in nature always gives me energy; I love running on a trail while listening to music, and I love dancing and yoga. That probably wouldn't work for you. I have to get into the habit, or else I get into the habit of not exercising. Being a TJ, maybe you just need to get your inner drill sergeant to yell at you to go to the gym at a certain time every other day? I don't know. Is there anything physical you find fun? Are there particular environments or people who give you energy? You could combine those into something physically active. Also I am a fairly active person. I work in an active job, and when I go to school or downtown I park far away and then walk/jog to wherever I'm going to avoid paying for parking; and I get exercise in the process. I'm motivated by the results a lot too; I love being fit, and feeling strong is really good psychologically. You could also try martial arts. That might be good for an ENTJ. Especially as you said you are a social worker- de-stressing I think would be important.
Oh yeah, and right now I'm taking a dance class, so I have to exercise at least once a week- classes are great for getting you in to the habit of exercising. Ideally you'll be motivated to exercise in between as well.