MR25 Ultra-marathon will take place end of this year. MR25 is the Macritchie Runners 25 (minutes for the 5K trail route) club. As long as you complete the 5K (men)/4.8K (for women) under 25 minutes, you are eligible to sign up as a member.
Prior to this, several long-distance marquee, running events will preface it:
a) Army/Bay Run (21K)
b) Northface 100 (50/100K)
c) Standard Chartered Singapore Marathon 2011