I'm 70 and reasonably fit but am concerned about altitude sickness on the Inca Trails. Is the risk higher on the short two day trail?

I'd say the Two Day Inca Trail is a much better choice if you're concerned about altitude. The highest point on the full (four day) Inca Trail is 4,215 metres but on the two day trail it's only 3,680 metres. That's still high, but on the two day trail you are at high altitude for much less time and you can sleep at lower altitude the night before. The full four day trek has you at about 4,000 metres for a long time, and that 4,215 metre high point isn't the only high pass you have to cross.

Obviously the longer you can acclimatise the better, I'd strongly advise against arriving and heading straight off onto the trek.

Most people acclimatise in Cusco, but at 3,500 metres the altitude sickness can be rough here too! I'd recommend spending a couple days before your hike in the Sacred Valley such as in Ollantaytambo, which is a much more manageable 2,792 metres.

None of this constitutes medical advice and if you have any concerns you should talk to a doctor first.