oooo... i volunteered with a mh charity and not only did i feel good about doing that i also learned loads - they have courses that volunteers can go on for free helped me understand myself a bit better.... the one i was with had kinda different type of volunteers depending on what you could commit to, so if is the same then i wouldnt fret too much bout the obligation - i just did what i could when i felt able to
if you do have a go, let us know how you get on (if that's ok) and good luck wit it all :)