If you're travelling a fixed distance, be very wary of cycling (and walking).

Some numbers:

 Casualty rate per 100 million
Casualties Occupant/ Occupant/ Occupant/
rider trips rider km rider hours
Killed
Pedal cycle 227 12.5 4.6 64
Walk 1753 7.0 6.6 27
Motorcycle 670 122 11.4 342
Car 2142 5.2 0.4 12.4
Bus and Coach 17 0.4 0.06 1.4
Killed and Seriously Injured (KSI)
Pedal cycle 4879 268 98 1377
Walk 17880 72 68 279
Motorcycle 12654 2311 215 6461
Car 29346 71 5.7 170
Bus and coach 892 14 2 51

Source: http://www.swov.nl/rapport/promising/wp5final.pdf which quotes Transport Statistics Great Britain 1979-1989

(If you're otherwise unhealthy and this is the only exercise you'd give yourself, you may still be better off. If you can make yourself do other exercise, please do.)

And if you thought you'd get away with walking by staying mostly not alongside traffic:

Table 2: estimated casualty risk, relative to crossing a street
Travel mode Distance travelled for casualty risk equivalent
to one road crossing
Walking alongside traffic, UK 160 metres
All walking, UK 210 metres
Road vehicle passenger, Australia 6,300 metres

Source: http://grapevine.net.au/~mccluskeyarundell/Ped_casualty.pdf