This is an operation they have been planning for years. The recent incompetence of Mossad and the IDF shows they either deliberately created a casus belli to attack Gaza, or are truly incompetent, both are bad.
There is no way you can "train" 300,000 deployed troops in open fields for URBAN warfare days before a massive urban mission. They are re-calculating to ensure they know what they're getting themselves into, as HAMAS today is not the same as before. It is much more organized and has more lethal and modern weapons.
There is no way they can neutralize the tunnels (as shown in US incompetence in Vietnam), it is too dangerous unless chemical weapons are used in which case the IDF exposes itself to chemical warfare, as do their citizens. I don't think they have an iron dome for chemical weapons, or do they?