Goodbye Limbs, Hello Water
... though they may be back if the brain can handwave drug-production and has a good diet.
You'll have to play with your parameters, but mushrooms be converted to a leather-like substance under the right circumstances. This is produced today and marketed as "vegan leather". Forming this into a glider, you might be able to make it to a water-body or at least lengthen your flight.
Using the mass of limbs for this and dropping some of the bones you can't convert, you could potentially splash down relatively softly, perhaps enough for the major organs to survive. Your glider/parachute is now a flotation device until your reach shore.
After that, survival is in the hands of rescuers, local wildlife eating the wreck, and the ability to feed. Photosynthesis won't be enough to power the brain, but eating wildlife might be.
Then, if you survive...
With the right drugs (and some handwaving to let the mushrooms know what to produce and where), it is possible to convert some stem cells into "embryonic" stem cells which can differentiate into arbitrary tissues. These cells can build an entire body naturally, so regrowing the lost limbs given the right nutrition can be handwaved by mushroom-dosed biochemistry - salamanders are able to regenerate lost limbs perfectly, which gives you a precedent.
Hunting pigeons with mushroom-made poisons or LSD could give this character enough to eat, in an urban setting without rescue. Smaller creatures could be more of a problem, and unfriendly humans would probably be.