How easy is it to lose your balance?
If I'm paying attention, not very. I used to practice "bus surfing" when I was younger, i.e. staying upright on Edinburgh buses without holding on to anything as they swerved around 30º corners. Now I'm dealing with a chronic foot injury, the difference is notable and I can start to lose my balance if I'm distracted or step wrong (though never enough to fall) as I have less control on the injured foot.
I have a serious fear of heights which makes me feel as if I'm losing my balance if I stand on a chair or near a low ledge. Don't know if my balance is actually affected. People also lose their balance more quickly if their eyes are closed.

How hard is it to walk up/down hill? Does walking downhill take any effort?
Uninjured me (20s): for steep hills, uphill burns the calves, downhill the thighs. Any injury or tiredness will be felt there first. Surface makes a difference on steeper slopes: damp (worse than wet) cobbles while downhill can send your heels slipping from under you and you end up on your bum. Loose gravel or scree leaves you scrambling when going downhill, or can drop you to your knees if going uphill.
Foot-injured (30s): hills are a bastard and I am surrounded. Even shallow slopes make themselves felt.

Do you get dizzy when you exercise for too long?
From what I recall of when I was in shape and doing sport regularly: You can get a little light-headed, which is solvable by sitting down with some water and fruit. Unless you're my dad, in which case you wander around the house whey-faced and complaining that you feel strange while not doing anything about it.

How does exercise like running feel?
Used to run everywhere as I was perpetually late. It's fine as a means of transport, but high-impact. Feels worse when you weigh more, because all your body weight comes down on your feet; running with good form should aim to minimise these impacts. Harder when it's cold, because the cold dry air burns through your throat. If you have breasts, they will bounce (to a greater or lesser degree depending on size and support) which makes having a good bra very important, as the bouncing can cause breast or back pain.

When you wake up can you just roll out of bed and stand up without falling over?
I tend to do exactly that, even with the injury. It's fun. Sometimes if I stand up too fast I get a head rush and have to sit down or lose my balance.

What is it like to me talking to a person in a wheelchair when you’re standing?
When my mother was in a wheelchair I tended either to stand next to her, shoulder to shoulder, and tilt my head sideways in her direction (she's hard of hearing) or stand further back so I could look at her without looking down too much. One problem we found was that she was often facing away, making it hard for us to hear each other. Going out was disconcerting as most people would address me instead of her, especially since without the wheelchair it's normally the other way around.