My workplace did challenges like this, and quickly learned you cannot do anything that gives awards for someone with the most hours/points. You either had people straight out cheating, with ridiculous amounts of miles/points, or the conversion of different activities made no sense. For example, during one myself and couple colleagues were training for a marathon, doing 40-50 miles of running a week, and somehow people doing a few 45 minute bike rides or elliptical sessions were ahead of us.
The only one that our workplace found that worked was to set a weekly goal, a popular guideline was something like 150-200 minutes of moderate exercise per week (usually meaning a brisk walk or greater level of effort for anyone between 18-64). If you hit that goal, you were put in a drawing for the week for a prize (like a $10 gift card or something). For people like me, who already worked out, we had a chance at a prize; and, for others it set the motivation to be active and get into the drawing.
Ultimately, don't over complicate it with what activity does what, or these miles equal this and those miles equal that. People will just get frustrated and not bother. The important part is just getting people to be active, however they can get it done.