Yes - blood tests is what the person who looked at my (admittedly dumb, no heart rate variability measure) resting heartrate suggested. Will make a GP appt. Previous bloods pre-op showed anemia. I've had them looked at and there was nothing weird, but that was before I started feeling broken.
In terms of diet.. I'm vegetarian. Lots of salads, quorn, tofu, beetroot, vegetables, seeds. Macros look pretty standard (60% carbs, 30% fat, 20% protein is goal. Lately protein has gone up at expense of carbs.
One weird thing is that I feel fine on hills - and can actually get my heart rate up in the 130s - 140s. It's the flats that kill me. My legs don't want to spin. Usually, i don't have any trouble getting my heart up in to the high 170s, low 180s if i'm chasing someone.
I'll think carefully about the 600 - might take the week off the bike and see how I feel.