Credibility of my Bike fit I have been an cycle enthusiast from my school days and I got a merida scultra 901 road bike recently.
However I am facing difficulties while I go over 60-70kms in a single go. Though i complete a 130km ride I end up getting a bad headache, neck pain and muscle cramps in my calf(when I rotate at high cadence).

In order to counter attack the neck pain, I did get a 60mm stem but the issue though mitigated has not resolved.

I'm 175 cms in height and I own a 52 cm frame size. I ride about 40 kms on my hybrid everyday without any problems.

Any info/guide for the above problems would be greatly helpful.
Thank you.
I would suggest your frame may be a bit small for you. I'm slightly shorter than you and I have a Scultura 906 in a 54 cm frame (size on the frame sticker says S/M). The longer stem may assist, however it may put you in a more race position which could lead to the neck pain. As for the calf cramps this could be a couple of things; 1) seat height too low, 2) lack of magnesium.

To check the seat height, hop on the bike and put the pedals at 12/6 o'clock. Rest your heel on the 6 o'clock pedal. If your leg is fully extended your saddle height is probably correct. If your knee has a bend then I'd suggest raising the saddle height until the bend is gone.

As for the magnesium, that comes down to trial and error.

On the neck pain, I'd suggest seeing if you can lift the stem (if your fork steerer is long enough) or maybe flip the stem to give yourself a little more height at the front of the bike.

As always, when doing a longer ride, make sure you're staying hydrated, as that is the only thing that causes me headaches when I ride a long distance. Too much heat, and not enough fluids. Let us know how you get on.