From 35 minute 5k to sub 20

It is hard to answer "how long"

It mainly depend on talent and dedication. But you are giving yourself for a big timespan at least, unlike people usually asking for 12 weeks plans. If you can stay in shape and if your body react well to training you will go quickly from 35 to 25 minutes, probably in less than 6 months, but be careful to not overtrain and injure yourself.

Once you have reached that point and that your muscles have become used to running, you can start to implement some speedwork. Running sub 20 demand speed, and it is harder for a woman than for a man. You may get there in 1.5 years with a lot of dedication and training, but you could also be stuck at 22-23 for some time and reach your goals only after entering your selected team.

Skinny fat? You know that 18.5 BMI is borderline underweight?

For avoiding the cramps, did you try to run very slowly?

For the muscle cramping, probably overstriding with too low of a cadence. Try running more steps per minute, with shorter stride length, and see if that doesn't help. A low cadence, and overstriding, results in more jarring up and down movement, with muscles flexed for too long. I'm no authority on this, it's just what worked for me.

Stop worrying about weight and BMI. You're already skinny. Eat when you're hungry, and possibly more often.

Cramps are caused by fatigue. The only thing that works to prevent that is to keep running, a bit more each week.

Don't wait 1.5 years to try cross country. Go out for track TODAY. Go talk to the coach. Run cross country THIS FALL. Prepare by running THIS SUMMER. Don't put it off.

Getting faster can be a long process, but it's not that complicated. Just keep pushing yourself a little bit more. No all out, not straining, not gasping in pain. Just a little bit more.

I started at 32 min at age 25 with the weight of 87kg.

After running average 40km a week for 2 years ( currently 27 y old) I now have a pb of 23:56 with the current weight of 73kg. I have no talent and probably doesn't train properly. You can do better than that :)

I noticed that you are a female. No one else mentioned it. Men stating their weight to you is not helpful. Medical doctors (for some reason) use B.M.I. Many drug stores and many grocery stores have percentage of body fat scales. High school girls often fret about weight and fat. Do not overly concern yourself with weight. Are you participating in spring T&F? If you get your 1600m time down to 5:45 or 1500m time down to 5:25 if you're European, you will race sub-20 5K XC most likely.