In the movie Pacific Rim, we see the beginnings of an alien invasion using very large mobile lifeforms.
Given that we want as large a lifeform as possible with the following traits:
It must be able to move under its own power at a minimum top speed of 45 kph for at least 2 minutes before needing to rest (reduce to indefinite duration speed). I.e. capable of outrunning a human on foot at maximum speed.
It must be able to move under its own power at 10 kph for a minimum of 48 hours, i.e. capable of outrunning a human on foot over the course of this time.
It must be able to live and function on the land surface of Earth for at least 14 days (absent hostile action) at full effectiveness. Bonus for being able to swim.
It must be grown to its operational size over any time period of 25 years or less, not constructed, though its first building block/cell/functional unit may be constructed/engineered. It may be gestated in one of its own kind as a parent or in an artificial womb, but must do the work of growing itself, provided with only raw materials and energy.
It must be capable of (and inclined to) inflicting significant damage to human infrastructure and populations, using non-biochemically-specific means (i.e. no toxins) over its full effective operational duration. If it is armed with projectile/incendiary weapons, it must be either equipped with sufficient magazine capacity to last 14 days of combat or be capable of in-the-field replenishment using found resources.
Evolution is not a concern. These are created beings, so as long as they can be grown from a single unit, they can incorporate design elements not available to merely evolved creatures.
It must be resistant to conventional (i.e. non-nuclear) human weapons of the current era (C2016), and must be able to absorb any practical amount of fire from standard man-portable firearms, and at least one direct hit from a tank-based weapon or at least one loadout of any conventional ordinance from any combat aircraft and still remain at least partially combat effective.
How big a lifeform could we realistically have, and in very general terms, what chemistry and body plan would it need to fulfil all of these criteria, if they can all be fulfilled? How (if at all) would it obtain additional energy for continued operations?
Are Pacific Rim's kaiju realistic?
Preference will be given to answers that can justify greater combat capability, longer operational durations, and a shorter production time for a given magnitude of body mass and volume.
Think out of the box here, guys. Don't feel constrained to CHO chemistry or vertebrate body plans. If some sort of Von Neumann nanotech made from metal and powered by atomic reactors can be justified, run with it. Should the body resemble a starfish or a centipede or a sea urchin or something else?