Based on the FAQ from their website, there will be bridging bus service to Stadium MRT
Free shuttle bus service from Stadium MRT station CC6 to Gardens by the Bay East at Rhu Cross <-----> (Return shuttle service between Stadium MRT & Gardens by the Bay East will run from 3:30pm to 11:30pm (on a first come first serve basis).
The race time for 21km/30km is now earlier at 6pm 👍. The previous (2018) starting time was 7:30pm.
Let's say if run at the worse timing of 4.5hrs (cut off time), still can finish the run by 10:30pm. So it is still not too late if need to catch bus/train home.
Should be good enough reasoning to sign up, unless those negative feedbacks (poorly lit route/run out of finishing medal/run out of finisher t-shirt size, etc) from previous Marina Run really put you off.