You're going to get a lot of advice, as you have begun to find out. If you're unfit as you say, just RIDE. Forget intervals right now, just pedaling for a while will give you a good workout.
Unless you happen to ride on very flat ground all the time, you're going to go out and find it's almost impossible to stay within the limits of what most call "base training". It's going to hurt, you're going to breathe hard, and you're going to sweat your A$$ off. You're also going to love it, get better (faster) very quickly, and if it is your goal...you're going to drop some body fat.
Someone told you "listen to your body". DO THAT, it will let you know when enough is enough and at the beginning it will probably do that pretty quickly. If you have a possibility of heart disease, etc. it would not be a bad idea to talk with your physician to get their thoughts.
I started riding ~3 months ago. My first ride was 6 miles, and I averaged a whopping 10mph. Granted I have a lot of hills here, but still nothing to be remotely proud of. That first week (rode every day) my goal was to actually be able to ride the hills without stopping to rest. I didn't make it..lol.
Two weeks after my first ride, I rode 44 miles (still stopped on some of the hills) and averaged 12.4 mph. I didn't move for two hours after getting home either, pretty exhausted. Took over 3.5 hours NOT including the almost 45 minute "lunch" break I took half way through.
Now, 2.5 months later, that same ride (if I work as hard as I did the first time) takes me a little over 2.5 hours at ~17mph average. I've since bought all these neat toys that let me know that ride has ~3000 ft. of ascent, power generated, etc. I also no longer have to stop. 😄
It's really only about now that I think I could actually go out and do what everyone terms "base training" which usually means lower power output, lower heartrates, etc. When I started riding, I couldn't stay balanced at the speeds it would have taken to keep myself within those limits. That being said, I STILL don't do the extremly low intensity "base training".
I also get all the intervals (for now) I need with the hills, my new thing is to attack them (the rolling ones anyway) and try to do whatever it takes to maintain speed over the crest.
Ric Stern (on this forum) is probably the best person to tell you about all this. He seems to suggest that to get adaptive changes in your body, you're going to need to exercise at higher intensities than what is normally recommended for "base training", OR you have to do a MASSIVE amount of training at the lower intensities. Based upon my own experience, I'd have to agree with him.
Bottom line is... ride as much as you can and have FUN!! 🙂
John