I would estimate between 60%-70%. The best strategy to use when attempting to survive for 70 days, is to try to avoid wounds. If the wounds that the Barbarian Prince receives are ever 1 less than his Endurance (fall unconscious), or greater than or equal to his Endurance you lose the game. This basically means that we need to avoid combat at all costs. The best way to achieve that is to hideout in a relatively safe location.
North of the Tragoth River
The first thing that is necessary to avoid combat is to cross the river. If you don't, at the end of each day, just before your evening meal you have a 50% chance (4/6) of being attacked by Mercenary Royal Guardsmen (e002). This encounter deals you on average about 1.71-1.55 wounds (depending upon if you Negotiate/Evade/Fight), and because it occurs about 50% of the time it also prevents you from Healing Wounds (r222) about 15% of the time. This is in addition to the normal event for the day that might prevent healing if combat or an escape occurs. If we stay North of the Tragoth River we will heal less than 0.85 wounds on average, but we will receive 0.86 (1/2 of 1.71) wounds on average plus any additional damage from other encounters.
Crossing the River
(I will fill in the percentages of crossing the river without dying at a later time. It is a little difficult to write up all the possibilities, because the strategy changes slightly based upon where you start)
Getting to the Mountains
After you get across the river, you need to head to the nearest mountain hex. Depending upon where you start, you should head to a different mountain range.
Ogon (0101): Head to 0103 mountain hex.
0701: Head to 0703, Barrier Peaks.
Ruins of Jakor Keep (0901): Head to 0703 Barrier Peaks.
1301: The worst starting position, head to 0703 or south to 1204.
Weshor (1501): Head south or west to the nearest mountain hex.
1801: You should already be in the mountains after crossing the river.
Hiding in the (safe) Mountains
The only way to receive wounds in the game, is through events and through combat. All events including combat are a function of the terrain that the Barbarian Prince is either resting in, or the terrain he was trying to move into before getting lost. Therefore, to minimize wounds, we only need to examine the likelihood of getting into combat for all 36 possible events for a terrain type and the amount of damage that you are likely to receive. Particular attention needs to be paid to encounters that prevent the Barbarian Prince from escaping (r220e), because escaping is one of the best tools for avoiding damage. Escaping is effective 50% of the time, but causes the prince's party to flee to a random adjacent hex (that isn't across a river). What is nice about this is, as long as we don't rest in the random hex we fled into, but instead travel back into the mountain hex then we will roll for an event on the relatively safe mountain encounter chart. I have included some of the safer terrains, and their percentage chance for combat, and percentage chance for a deadly inescapable combat.
Countryside: 10.74% combat, 2.83% lethal* (3x Wolves, 1x Goblin Keep)
Forest: 10.01% combat, 1.54% lethal* (1x Wolves, 1x Spectre)
Hills: 6.11% combat, 0.79% lethal* (1x Wolves, 1x Dragon)
Mountains: 2.82% combat, 0.39% lethal (1x Goblin Keep)
*Note: Forest, Countryside, and Hills all have at least one Great Hunting Cat encounter that prevents the victim from escaping. I didn't include this within the lethal calculation, because it doesn't occur that often for most terrains and with a combat encounter percentage of 10% or less you will usually be healed up enough to survive the encounter.
Wrapping it All Up
If you can make it to the mountains, you only have to survive the remaining days. This will usually mean that you will be escaping from combats (if they occur), and then attempting to travel back into the mountains to rest on subsequent turns. You will likely be suffering from starvation for much of the game, but since starvation only affects Combat Skill and Carrying Capacity (not escape chances), it shouldn't have much of an effect if you are only trying to survive. To figure the chances of not having a lethal encounter after X days, just take 1-(lethal encounter%) and raise it to the DAYS power. The result is about 76% chance. If you figure in the river crossing, the time to get to the mountains, the true survival percentage is probably a little bit lower. I would estimate a 60%-70% chance of surviving (with the above strategy).