Translation:He decides to bike instead of walking.
The English translation is not correct. One would say "he decides to bike instead of walkING"
I've heard conflicting reports on this from native speakers. Perhaps there are regional differences?
It's a tricky issue since there is a rule that prepositions should be followed by a noun and hence people think the gerund should be used. Also it seems hard to find this exact rule stated in a dictionary etc.. However I did come across these phrases in a dictionary.
- "I was forced to stop training for shows and I began to hobble instead of walk." - https://en.oxforddictionaries.com/definition/hobble
- "I'm trying to focus and not to false start, fall at the line or basically walk instead of run out of the blocks as I sometimes do because any of these things could happen." - https://en.oxforddictionaries.com/definition/block
Hence it seems more complicated then assuming the gerund needs to be used.