We recently had a similar situation with our 2006 R-25. We were unable to shift into reverse before the throttle opened, revving the engine.
Troubleshoot the issue yourself with the engine OFF.
The first thing to do is disconnect the shift cable from the transmission shift lever at the rear of the engine. Once it's disconnected, physically shift the transmission by lifting, then pushing the lever. This should help you determine if there's a problem with the transmission. If it shifts easily, proceed to the next step.
Second (best done with another person), hold onto the transmission end of the shift cable while your helper slowly moves the helm control lever forward and reverse. If the cable binds, check for kinks in the solid rod where it exits the cable sheath. Release the cable sheath from the bracket and lubricate the cable inside the sheath and clean any rust or rough areas with 220 grit sand paper.
If those steps don't solve the problem, or there is a problem with shifting the transmission (step one), phone Andrew or Kenny.