From parking lot to parking garage it's more like 30 minutes depending on the traffic. It takes time to get out of the lots and garage plus it could take you 15-20 minutes just to get from the MK to the parking lots...if not longer. I think you should allow an hour at least to make sure you can make it.
From RPR car park it took us around 25-30 mins to get to MK, that was using Turkey Lake Road as opposed to I4. Then you have to get to the park, so agree with Metro give yourselves around an hour to get in the park.
During our April spring break week we did a day at WDW. From the Hard Rock parking lot to DHS parking lot it took about 35 minutes (we went I4). From there we had to take a tram to the gate and walk in. That probably took another 15 minutes. I would definitely give yourself an hour.