I think it is an excellent idea to be a volunteer for the first time somewhere large. They usually have a Volunteer Co-ordinator or a similar name. They will be familiar with what's it like to be a volunteer. As they say, try various things, within the same organisation. If they like you they might be willing to give you a reference (after you have been there a while) or a job might come up, for holidays etc.
You could explain about the cycling and ask if you could do Saturdays or Sundays, but not both. As a volunteer if you have a long Club cycle ride that you defiinitely want to do, let them know well in advance.
Well done, and good luck.
Just two small pieces of advice. Work hard by all means, but do not exhaust yourself. Take the breaks that you are offered, and have a proper lunch break. Ask questions if you are not sure what they are expecting you to do. You could ask the young lady, who has also recently been a volunteer.
Btw, just out of interest, did you find this on the Do-it.org site?