Traiing on a too big raodie....questions

You are asking for trouble riding a bike that is way too big for you.  It is way too long for you and probably to too high also.   You will overreach while riding, causing problems in your lower back and shoulders, maybe even your elbows.  You might be able to replace the stem but it would have to be something along the lines of 70mm and may still not solve the problem!?! Why take the chance?  Do yourself a favour and ride your  P2C on all your rides, long or short!  

Sell the road bike on eBay or Craigslist and buy another  one with the money you get. 

as long as you can standover it, you can probably make it work with a new short stem.   The top tube difference on a 58 vs a 61 is probably 1 inch. 

My question is why would you want to ride a non-fitting old Trek when you have a nice Cervelo that fits?

Just make some appropriate adjustments if you feel problems.  Assuming the crank arms are a similar length and the pedals are similar, measure each bike from the center of the crank on the non-drive side straight up the seat tube and seat post to the top of the saddle along that line to make sure that there aren't any major differences in saddle height (like 1.5" or something).