SVP m'expliquez

Question

C'est bien d'y aller à pied, where does the 'd' before 'y' come from

Accepted Answer

Susan, this is an impersonal expression using 'ce' as a 'dummy subject' (that is 'ce' does not replace a known noun - it is a general statement).  
The preposition 'de' is needed whether using 'il' or 'ce' as dummy subjects, in impersonal expressions of the form "il est (adjective) de (infinitive)"  or "c'est (adjective) de (infinitive)".  
This, and the distinction between 'real' and 'dummy' subjects, is discussed in section 2 of the attached link from Laura Lawless.
 https://www.lawlessfrench.com/grammar/using-impersonal-expressions/