Shit battery imo, easy to work out your current consumption though so you know exactly how long it will last, V/I X R.
Having the engine running will only slow down the effect, but you will still be taking out more from the battery than the alternator can supply when idling if it's gone flat in 30 minutes, don't forget a 70 amp alternator does not charge at 70!!, this is a peak reading, and only has to do it for 1 second, as a rough rule no more than 1/2 the rated capacity will be delivered continuously, but for short times up to around 70%.
Deep cycle battery will do nothing to help the cause, as these are only advantageous when depleted in that no damage is caused, unlike a std lead acid battery.
You ideally need a AGM battery, these have vastly better performance, but at a price, around £120 minimum, would most likely be better to just get the biggest AH one you can in the tray, or even fit extra one in the boot etc.
tabetha