Thanks for the replies, so glad it's not just me. Blackbird, you've described it perfectly!
I felt like a bit of an idiot, spluttering with a cup of water, but it felt really odd. I did slow to a walk, and it still felt difficult.
I don't want to have to carry a water bottle, little things like that tend to put me off quite a lot. Looks like I'll have to add drinking practice in if I start going any further! Thanks for the tips.