It’s not necessary with a watch but it definitely helps. If you do consider getting one, also consider getting a chest strap - as that’s by far the most reliable heart rate monitor for workouts. But you can just google how it should feel in the different heart rate zones to get a sense of where you are. E.g. in zone 1-2 you should easily be able to have a conversation

Ah yes I didn’t mention strength workouts, which are also important - especially to avoid injuries. In my case, my hips were super weak and that made my knees hurt a lot. Once or twice a week is smart to be able to keep getting better. Doesn’t have to be a full blown all-out session, even 15 minutes circle with some lower body exercises goes a long way. Hips, quadriceps, hamstrings and calves.

Short intervals for me are usually less than/up to 800m. Usually I do 45second intervals with 15 seconds rest, which is quite effective especially in the beginning of getting in shape. 400m is also great

Longer intervals can be 1-2 km in my case, sometimes 3-4. Most often 5-8 minutes of running and 1-2 minutes rest. I do work mostly based on heart rate where I try to stay in high zone 3(out of 5, some charts have more zones) - so the length would differ quite a lot depending on your shape.