I'm a lead in a film with no experience!

So basically, I heard there is an Independent, Full Length Feature Film being filmed near where I live. As I've studied in Film Production, I wanted to get involved in the film as a runner/roadie or an assistant to help out with everyday things onset. However, I went to the 'Casting & Crew Day' and explain I wanted to get involved with the production as a runner etc, then the director of the film turn to me and said "You know, we are looking for a male lead, and you have the look we're going for... would you be interested?" (Baring in mind I've never acted before and I told them this!)

I was a bit hesitant at first but ended up saying Yes, and spent the day reading lines with the producer and some of the other cast. I must've done something right as they still want me as the lead!

Now a week or two has passed, I've started to worry about what I'm getting myself in for. I'm actually excited about being in a film but I'm worried I'll let people down.

I'm just asking for some advice about being a lead in a film and also acting in general. Any advice would be greatly appreciated!!