Footbridge? 25 feet? Piece of cake.
You don't need steel for that. Pressure-treated wood is just fine. If you can't get 2x12s that are 25 feet long, just laminate three or four together with the ends staggered where they occur in the overall length. (One joint near the middle, one joint at the quarter point, another joint at the three-quarter point.)
Two old carpenters' rules of thumb for floor joists (spaced 16" on center) are:
(1) Take the depth of the joist in inches, multiply by 1-1/2, and that's the span in feet. (i.e. a 2x12 would safely span 18 feet).
or
(2) Take the depth of the joist in inches, subtract 2, then double it for the span in feet. (i.e. a 2x12 would safely span 20 feet.)
Both of those are for supporting uniform floor loads in a room, complete with people and furniture. A section of floor 4 feet wide (which is a useful width for a foot bridge) would have four joists under it. If your bridge had three beams, each made up of two or three 2x12s laminated together, you'd be in good shape.