Be careful with that tho Stevie, 15mins working twice as hard will be more effective, make sure it's half hr of STRENUOUS exercise and not a simple plod

A tip for the rower, do 2000m and see how long it takes, that's the standard distance for indoor rowing championships and done hard is extremely tough. Anywhere near 8mins is very good (2:00/500m split time) but sub 10mins would be a start