Possible to do 5k followed by marathon (next day)

Yes.  Piece of cake.  I've had athletes who raced double triathlon events - a sprint tri on Saturday and half ironman on Sunday.  Back to back running races is pretty straightforward.  Depending on your fitness level, you may need to hold back a bit on the 5k, and focus on recovery between the races.

I think you will be fine going hard on the 5K and doing a Marathon the next day.  One year I registered for some 1/2 marathons as training days for a full marathon I was planning to do.  One of the 13.1's hit at the end of a heavy training week.  So with no taper and started the half marathon.  The first 6 miles I felt very dead legged due to the volume I had done over the previous 10 days.  at around the 6 mile mark all the lactate had worked out of my legs and I started to feel loose.  The last 7 miles were as good as I have ever run a half marathons. So...you may feel your 5K for the first part of the Marathon, but once you get going will probably be fine.

I did the 5K five days after a Marathon once.  A lot of guys in my marathon training group had done 5K's the week after their first Marathon and though I needed to have that experience too so they practically signed my up for the 5K.  The 5K felt great for about the first 2 miles then I fell apart and finished about 10% slower than a typical 5K (so I was about 30% slower on that last mile).