So in the Table ‘Challenge’ I have the field ‘Steps’ which has one entry of 2,000,000. This is the number I want every step recorded by any user to be taken away from.

So from what you are saying that would be the ‘goal steps’ and then ‘do a search’ of the steps table sum and subtract. But how do I physically do it?

If want a global result you can do the same get the challenge steps and subtract from sum of steps completed. You can do that using dynamic data from database, directly in the field that do you wantto show the data.