Can I use old abandoned iron pipe as electrical conduit?
Your biggest concern here is going to be the condition of the interior.  If they are at all corroded or otherwise rough on the inside they run the risk of damaging the insulation, which can lead to an eventual short and/or electrification of the pipes.  Protecting the wire where it enters and exits the pipe is also a concern.

Basically I think you could do it, but I wouldn't recommend it.  My gut feeling is that it's  too risky.

Since you've got a perfectly straight run - can you possibly remove them and replace them with proper conduit?