I follow the ross book as my bible, but it wont make sense if you're new to fitness/dieting/training.

When i switched from bodybuilding to boxing, it was a big transition in order to get into shape,

1) Me being an asthmetic
2) Coming from bodybuilding meant i never ran or done shit on a regular basis.

Before you start anything, its always good to build a foundation.

Sprints will get you up to scratch faster than anything.

For example
Sprint 100m
Walk 100m

Do that for 20-25min (This is all done in 1 session, there is no break, in other words your 100m walk is your break, so its best to drink water durin that period), and that can be your session for the day.

Do that 3 times a week, and the other 2 days can be a jog (light run) for 1-2km, or whenever you feel that you are extremely fatigued, but i recomend its best to set a distance, that way you can push yourself to reach that goal.

Anyway, within a month you'll be fit, when you feel the jogs are getting too easy you can replace them with the sprint workout i just gave u, or you can just increase the distance by another kilometre or 2.

After all this is done, im more than sure you will be able to maintain a session in boxing with other people who are 'fit'.

Hope it helps out, it did with me.

Peace