How difficult would it be to maintain 17.5 Mph for an hour? Whats a good target speed to maintain inside a velodrome for an hour? I believe Bradley Wiggins maintained an average speed of around 35 Mph for an hour, how difficult would it be to maintain half that speed for a similar amount of time?
The short answer is: not that difficult. The long answer requires some explanation.

The equations of motion for a rider on a bicycle are well-understood if not always well-known. The power needed to propel a bicycle on firm flat ground (as on a velodrome) varies approximately with the cube of speed. Thus, to double your speed, you would need to increase your power output by approximately a factor of eight; riding at half-speed decreases your power output to approximately one-eighth of full-speed power. 

As may be obvious, Brad Wiggins was highly trained and highly optimized in both equipment and position to minimize his coefficient of rolling resistance (Crr) and aerodynamic drag area (CdA). That means even at 17.5 mph he could very possibly use less power than most cyclists would. 

That all said, this bicycles.stackexchange answer discusses the equations of motion for a cyclist, from which you could calculate the power required to ride at 17.5 mph on a velodrome using reasonable estimates of Crr and CdA. For most average-sized adults, not highly optimized for position, a reasonable estimate of CdA on a standard pursuit bike similar to but not identical to the one used by Wiggins would be around 0.275 m^2. For this example, then, one could cover 17.5 miles on the velodrome at right around 100 watts of output. In comparison, an average sized man, walking on firm flat ground, would expend roughly 75 watts. So riding a velodrome at 17.5 mph would be more difficult than walking at normal speed for an hour but not horrendously more difficult. However, because of the cubic relationship between speed and power, riding at 20 mph would require nearly a 50% increase in power over riding at 17.5 mph, and riding at 25 mph would require a trebling of power. Many non-sedentary adults can produce somewhere between 2.0 and 3.0 watts/kg for an hour, so such a 75 kg man would be able to maintain between 150 and 225 watts for an hour. As you can see, many (but not all) adults would be able to maintain 17.5 mph, but not as many would be able to maintain 20 mph, and relatively few would be able to maintain 25 mph.