there are probably multiple reasons for this:
- The amount of Japanese people learning Mandarin is probably quite high, because of the low distance between those countries and China being the biggest power in that region, so there is just more material and more often means better.
- Even if langauges are not related you will sound many similarites, especially in the kanji related parts and those will help you. Also both languages are very different from English in the way they work regarding conjugation, declination, particles etc. but many concepts you will need in mandaring (e.g. particles) will already be covered in your japanese knowledge.
- Those similarities will allow a different approach to the langauge.
Some scientific date related to this: The average time needed to reach a level of B2/C1 (don’t know exactly anymore) in Japanese when already knowing a kanji using langauge will likely reduce by 1000 hours, being about one third of the total time you need. (Sorry, I can’t find the source anymore, so no reliable info
)
good to hear 



