It feels different at every stage of its evolution, but once it really is established, I think it feels like the brake pad on the bicycle is rubbing on the tire and one has to pedal against this friction all day long. You can do it but knowing that it's going to be a lot of work to maintain forward motion, you tend to become apathetic toward any kind of movement. At the end of the ride, you are physically worn out. You are mentally worn out because you know that it will be this way (or worse) the rest of your life.
I'd say feeling of extreme fatigue is dreadful when you first wake up from a nap. This is the moment you realize that this disease is serious; resting your body does not equate to what we usually expect to be recovery and a state of peacefulness.
I'd say the feeling of outright apathy is the hardest thing I have recently experienced. It's so bad that I physically can't move because my body is not able to supply any dopamine due to the mental status of "being apathetic". Normally we have a task to do (i.e. scratch my arm because I feel an itch), the brain reacts with a calculation of how much dopamine we will receive for performing the given action, and in some cases may choose to not make the move because the effort is not worth the reward (squirt of dopamine and a relief from the itch). But realize that the apathy also will be part of the over-all decision to perform the movement. Apathy might actually be created because mentally you know that there is very little or no reward for taking any physical or even mental action.
What does the Parkinsons feel like before the medications are introduced but the disease has established itself?
Feeling we are losing control and that things will be worse in the future, adds stress & anxiety which creates a destructive feedback loop.
Parkinson's Disease: emerging treatment target, the NLRP inflammasome. 
Sesaminol prevents Parkinson's disease by activating a signaling pathway
PD Mechanics: Cause of Resting Tremors
